2014 Littelfuse, Inc. Specifications are subject to change without notice. Revised: 12/14/14 Teccor brand Thyristors AN1007 120 V rms (170 V peak) V P- 1 V rms or 1.6 V peak MAX V P + V T + V T- Figure AN1007.3 Waveform Across Static Switch A typical example would be in the application of this type circuit for the control of 5 A resistive load with 120 V rms input voltage. Choosing a value of 100 for R 1 and assuming a typical value of 1 V for the gate to MT1 (V GT ) voltage, we can solve for V P by the following: V P = I GT (R L + R 1 ) + V GT Note: R C is not included since it is negligible. V P = 0.025 (24 + 100) + 1.0 = 4.1 V Additionally the turn-on angle is 4.1 = sin -1 = 1.4 O 170V PK The power lost by the turn-on angle is essentially zero. The power dissipation in the gate resistor is very minute. A 100 , 0.25 W rated resistor may safely be used. The small turn-on angle also ensures that no appreciable RFI is generated. The relay circuit shown in Figure AN1007.1 and Figure AN1007.2 has several advantages in that it eliminates contact bounce, noise, and additional power consumption by an energizing coil and can carry an in-rush current of many times its steady state rating. The control device S 1 indicated can be either electrical or mechanical in nature. Light-dependent resistors and light- activated semiconductors, optocoupler, magnetic cores, and magnetic reed switches are all suitable control elements. Regardless of the switch type chosen, it must have a voltage rating equal to or greater than the peak line voltage applied. In particular, the use of hermetically sealed reed switches as control elements in combination with Triacs offers many advantages. The reed switch can be actuated by passing DC current through a small coiled wire or by the proximity of a small magnet. In either case, complete electrical isolation exists between the control signal input, which may be derived from many sources, and the switched power output. Long life of the Triac/reed switch combination is ensured by the minimal volt-ampere switching load placed on the reed switch by the Triac triggering requirements. The Thyristor ratings determine the amount of load power that can be switched. Normally Closed Circuit With a few additional components, the Thyristor can provide a normally closed static switch function. The critical design portion of this static switch is a clamping device to turn off/eliminate gate drive and maintain very low power dissipation through the clamping component plus have low by-pass leakage around the power Thyristor device. In selecting the power Thyristor for load requirements, gate sensitivity becomes critical to maintain low power requirements. Either sensitive SCRs or sensitive logic Triacs must be considered, which limits the load in current capacity and type. However, this can be broader if an extra stage of circuitry for gating is permitted. Figure AN1007.4 illustrates an application using a normally closed circuit driving a sensitive SCR for a simple but precise temperature controller. The same basic principle could be applied to a water level controller for a motor or solenoid. Of course, SCR and diode selection would be changed depending on load current requirements. 1000 W Heater Load 120 V ac 60 CPS D4015L CR 1 -CR 4 CR 4 CR 3 CR 1 CR 2 S4010LS2 0.1 F R 1 510 k SCR 1 Twist Leads to Minimize Pickup Hg in Glass Thermostat Figure AN1007.4 Normally Closed Temperature Controller A mercury-in-glass thermostat is an extremely sensitive measuring instrument, capable of sensing changes in temperature as small as 0.1 C. Its major limitation lies in its very low current-handling capability for reliability and long life, and contact current should be held below 1 mA. In the circuit of Figure AN1007.4, the S2010LS2 SCR serves as both current amplifier for the Hg thermostat and as the main load switching element. Thyristors Used as AC Static Switches and Relays (continued)